Rear projection screens and light diffusers are light filters which provide an optically diffusing medium for transmitting light from an image source (image light) on one side thereof to a viewer on the opposite side thereof. A conventional single-layer refractive light filter has been described in U.S. Pat. No. 2,378,252, which includes a refracting lens system as its principal component. The refracting lens system comprises an array of spherical glass or resin beads embedded in an opaque binder layer, and is mounted on a transparent support material. Typically, the light filter is oriented with the bead layer toward the image source and the transparent support material toward the viewers. U.S. Pat. No. 3,552,822 discloses a light filter which also includes both an anti-reflection coating and a bead layer oriented away from an image source.
The opaque binder layer serves a number of purposes, including affixing beads to the support material, reducing the reflectivity of the light filter, and reducing the amount of light transmitted through interstices between the beads of the lens system. Light from an image source is refracted by the beads and dispersed to the viewer through a plurality of transmission areas of the beads. These transmission areas each comprise a point of contact between one of the beads and the support material and an area surrounding this point where the opaque binder layer is too thin to absorb substantially the refracted light. As used herein, dispersion refers to spreading light intensity over a range of angles irrespective of its wavelength, and is not limited to variable spreading of light intensity as a function of its frequency or wavelength.
Rear projection screens and light diffusers are characterized by their gain, contrast, resolution, transmittance, dispersion, and ambient light rejection. These properties are determined in part by the structure and composition of the component materials. For example, the gain, which is a measure of the intensity of transmitted light as a function of the viewing angle, is determined largely by the difference in the indices of refraction of the spherical beads and the surrounding media. Similarly, the ambient light rejection and contrast are determined largely by the opacity of the opaque binder layer, and the resolution is determined largely by the size of the beads used in the lens system.
The interdependence of these optical properties, and their dependence on the properties of component materials, limit their simultaneous optimization in conventional single-layer light filters. For example, if the opacity of the opaque binder layer is increased to enhance the ambient light rejection of the light filter, transmission of refracted image light through the transmission areas of the beads will be reduced. Also, with conventional single-layer light filters, a greater percentage of image light is reflected off back surfaces of the beads and lost, relative to light filters of the present invention. The range of indices of refraction of available materials also limits the performance of single-layer light filters.
Multi-layer refractive light filters have been described in U.S. Pat. No. 5,563,738. These light filters include a refracting lens system as their principal component, and further include an additional optical layer or layers for varying the optical properties of the light filters, and for providing improved simultaneous control of image transmission, ambient light rejection, contrast, and gain.
Single-layer and multi-layer light filters which include an array of light transmissive beads typically also include irregularities in the spacing of beads within the array. Such irregularities are formed during manufacture as a result of imperfections in the manufacturing process, and also as a result of the tolerances of component materials. The irregularities typically occur on a somewhat random basis which can appear quasi-periodic, and can give light filters a grainy appearance. Such light filters can exhibit reduced transmittance in front of gaps between the beads caused by the irregularities.